Oil and natural gas are extracted from underground formations by drilling boreholes to reach hydrocarbon-bearing zones. Steel tubing (“casing”) is inserted into the borehole, after which cement is pumped into the area between the casing and the borehole wall. The casing and cement prevent the borehole from collapsing under overburden pressure. Production tubing is inserted into the casing to convey the oil and gas to the surface. Sand screens within the casing prevent the ingress of fine rock debris into the well. Collectively, these parts of the well are designated as “the completion.”
Various sensors are utilized in oil and gas wells. In order to help improve the productivity of hydrocarbon-producing wells and enhance the recovery factor of reservoirs, it is known to monitor both the motion of the fluids present in the hydrocarbon-bearing zone and other parameters affecting the operation of the completion. In order to monitor these parameters it is desirable to place sensors within the well and also some distance away from the well in the surrounding formation. The sensors measure local physical properties such as pressure, temperature, electrical resistivity, fluid flow rate and fluid composition. Sensors may also be deployed in arrays to detect seismic waves generated by sources located either within the well, within adjacent wells, or at the surface, for the purpose of delineating fluid fronts. Modern wells also incorporate equipment to provide zonal isolation and flow control in separate producing zones, in the form of packers and valves. It is desirable to also monitor the proper operation and health of these elements by embedding sensors in them.
The current trend is a significant increase in the number of sensors in oil and gas wells. With the advent of horizontal drilling it has become possible to expose a relatively long section of the well to the hydrocarbon-bearing zone. In particular, the zone exposure may be kilometers in length. As already described, sensors should be distributed along the hydrocarbon-bearing zone to effectively monitor the behavior of the reservoir. As a result of increased zone exposure, increasingly large numbers of sensors are being installed in wells. In the future, a single well could conceivably incorporate hundreds of sensors dispersed over a substantial volume around the borehole. However, exchanging communication and power with such large numbers of sensors may be impractical with current technology.
It is known to exchange of information and power between devices located on the production tubing and the surface by running electrical wires or optical fibers along the tubing. Significant amounts of power, e.g., hundreds of watts, and high data rates, e.g., hundreds of kilobits per second, can be delivered downhole by this method. Unfortunately, this conventional technique is relatively ineffective at linking devices located on the tubing to sensors located on the fixed parts of the completion, in spite of the relatively short distances involved. Consequently, the sensors, wires and hydraulic lines are often placed on the outer surface of the casing and cemented in place. However, this solution presents many drawbacks. For example, it is only applicable to new wells, it does not allow repairs after installation, and it interferes with the cementing process, frequently leading to a lack of integrity and sealing capability of the cement column.
It is generally known to implement wireless communication for networks of small sensors by means of electromagnetic fields. For example, RF communications are utilized with RF-ID tags for monitoring conditions in buildings. However, such techniques are not practical for the downhole environment because the electrical conductivity of most formations strongly attenuates these fields and hampers their propagation. The presence of a metallic casing, liner or sand screen further degrades communication between devices located inside the borehole and devices located within the formation. Several techniques have been proposed to address the problems. For example, Ciglenec et al. in U.S. Pat. No. 6,070,662 discloses communicating with a sensor shot into the formation by incorporating a miniature battery in the sensor to power a transmitter. However, such a battery has a limited life, perhaps providing only a few days of operation. Aronstam et al. in US patent application publication 2005/0011645 describes small data carriers flowing with the wellbore fluids, which are either pumped continuously from the surface or released from a magazine located downhole. However, the data throughput is relatively low in both cases. Salamitou et al. in US patent application publication 2004/0238166 discloses a miniature sensor which is inserted into a casing hole and remotely powered and interrogated by a device located in the wellbore. However, the sensor must be mounted flush with the casing wall. Gao et al. in US patent application publication 2005/0055162 describes a wireless network of extremely small sensors pumped in fractures. However, it relies on radio waves and thus is limited in its range and cannot operate practically through steel casing. All of such references are herein incorporated by reference.
Despite being the focus of considerable research and development, the large-scale use of sensors downhole is still hampered by two main technical difficulties. One technical difficulty is the exchange of information between the sensors and the surface, and possibly between the sensors themselves. The other technical difficulty is the delivery of suitable amounts of power to the sensors. There is therefore a need for techniques which are not subject to the limitations of currently known techniques for transmitting information and power between a device located in the wellbore and sensors deployed either in the wellbore, within various components of the completion or on their surface, or within the formation some distance away from the borehole.